1) Everyone's doing it.
2) It separates the business logic from the JSP.
3) You have to type less.
4) Its fun and powerful.
5) Web designers can learn it too.
6) Its on the exam.
7) You will know more than your peers.
8) Your JSP code will be clean and easier to read.
9) Your boss will make you his "go to" guy.
10) You might even get a raise.
As for when to use it? You should use it when you have J2EE 1.4 implemented, since you need JSP 2.0. But you don't have to use EL. Its another way of doing things, its all upto you...